# Type alias, alias template (since C++11)

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Type alias is a name that refers to a previously defined type (similar to `typedef`).

Alias template is a name that refers to a family of types.

### Syntax

Alias declarations are declarations with the following syntax:

`using` identifier attr﻿(optional) `=` type-id `;` | (1) |
`template` `<` template-parameter-list `>` `using` identifier attr﻿(optional) `=` type-id `;` | (2) |
`template` `<` template-parameter-list `>` `requires` constraint `using` identifier attr﻿(optional) `=` type-id `;` | (3) | (since C++20)

attr | - | optional sequence of any number of attributes
identifier | - | the name that is introduced by this declaration, which becomes either a type name (1) or a template name (2)
template-parameter-list | - | template parameter list, as in template declaration
constraint | - | a constraint expression which restricts the template parameters accepted by this alias template
type-id | - | abstract declarator or any other valid type-id (which may introduce a new type, as noted in type-id). The type-id cannot directly or indirectly refer to identifier. Note that the point of declaration of the identifier is at the semicolon following type-id.

### Explanation

1) A type alias declaration introduces a name which can be used as a synonym for the type denoted by type-id. It does not introduce a new type and it cannot change the meaning of an existing type name. There is no difference between a type alias declaration and typedef declaration. This declaration may appear in block scope, class scope, or namespace scope.
2) An alias template is a template which, when specialized, is equivalent to the result of substituting the template arguments of the alias template for the template parameters in the type-id.

    template<class T>
    struct Alloc {};

    template<class T>
    using Vec = vector<T, Alloc<T>>; // type-id is vector<T, Alloc<T>>

    Vec<int> v; // Vec<int> is the same as vector<int, Alloc<int>>

When the result of specializing an alias template is a dependent template-id, subsequent substitutions apply to that template-id:

    template<typename...>
    using void_t = void;

    template<typename T>
    void_t<typename T::foo> f();

    f<int>(); // error, int does not have a nested type foo

The type produced when specializing an alias template is not allowed to directly or indirectly make use of its own type:

    template<class T>
    struct A;

    template<class T>
    using B = typename A<T>::U; // type-id is A<T>::U

    template<class T>
    struct A { typedef B<T> U; };

    B<short> b; // error: B<short> uses its own type via A<short>::U

Alias templates are never deduced by template argument deduction when deducing a template template parameter.

It is not possible to partially or explicitly specialize an alias template.

Like any template declaration, an alias template can only be declared at class scope or namespace scope.

The type of a lambda expression appearing in an alias template declaration is different between instantiations of that template, even when the lambda expression is not dependent.
    template<class T>
    using A = decltype([] {}); // A<int> and A<char> refer to different closure types
| (since C++20)

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_alias_templates` | `200704L` | (C++11) | Alias templates

### Keywords

`using`

### Example

Run this code

    #include <iostream>
    #include <string>
    #include <type_traits>
    #include <typeinfo>

    // type alias, identical to
    // typedef std::ios_base::fmtflags flags;
    using flags = std::ios_base::fmtflags;
    // the name 'flags' now denotes a type:
    flags fl = std::ios_base::dec;

    // type alias, identical to
    // typedef void (*func)(int, int);
    using func = void (*) (int, int);

    // the name 'func' now denotes a pointer to function:
    void example(int, int) {}
    func f = example;

    // alias template
    template<class T>
    using ptr = T*;
    // the name 'ptr<T>' is now an alias for pointer to T
    ptr<int> x;

    // type alias used to hide a template parameter
    template<class CharT>
    using mystring = std::basic_string<CharT, std::char_traits<CharT>>;

    mystring<char> str;

    // type alias can introduce a member typedef name
    template<typename T>
    struct Container { using value_type = T; };

    // which can be used in generic programming
    template<typename ContainerT>
    void info(const ContainerT& c)
    {
        typename ContainerT::value_type T;
        std::cout << "ContainerT is `" << typeid(decltype(c)).name() << "`\n"
                     "value_type is `" << typeid(T).name() << "`\n";
    }

    // type alias used to simplify the syntax of std::enable_if
    template<typename T>
    using Invoke = typename T::type;

    template<typename Condition>
    using EnableIf = Invoke<std::enable_if<Condition::value>>;

    template<typename T, typename = EnableIf<std::is_polymorphic<T>>>
    int fpoly_only(T) { return 1; }

    struct S { virtual ~S() {} };

    int main()
    {
        Container<int> c;
        info(c); // Container::value_type will be int in this function
    //  fpoly_only(c); // error: enable_if prohibits this
        S s;
        fpoly_only(s); // okay: enable_if allows this
    }

Possible output:

    ContainerT is `struct Container<int>`
    value_type is `int`

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
CWG 1558 | C++11 | whether unused arguments in an alias specialization participate in substitution was not specified | substitution is performed

### See also

`typedef` declaration | creates a synonym for a type
namespace alias | creates an alias of an existing namespace

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